Assembling electronics often comprises mounting various electronic components (e.g., transistors, capacitors, resistors, semi-conductor components, etc.) on an electronic substrate (e.g., a rigid or flexible circuit board). Often, these substrates are, in turn, mounted on or connected to other components and devices. For various reasons, traditional mounting methods utilizing lead-based solder have become less desirable. Increasingly common now are the use of conductive polymeric adhesives to mount electronic components onto electronic substrates, and non-conductive polymeric adhesives to connect various components and devices.
In general, an uncured conductive adhesive is applied to an electronic substrate so that, when placed within the adhesive, the electronic component is connected to the underlying circuitry. After the electronic component is placed in the adhesive, the adhesive is cured to securely connect the component to the substrate. Similarly, an uncured non-conductive adhesive may be applied to a microelectronic substrate, the substrate placed on another component or device, and the adhesive cured to securely connect the substrate to the component.
Various methods of curing polymers are known. These methods typically comprise the application of heat by conventional techniques. Unfortunately, many electrical components are susceptible to damage from elevated temperatures. Consequently, great care must be taken during the curing process to avoid damaging the electrical component, the substrate, or both from the applied heat. Additionally, many different electrical components may be attached to a substrate, some of which may be more susceptible to elevated temperatures than others. Furthermore, it may be difficult and expensive to heat a selected area of an electronic substrate without exposing the surrounding area to somewhat elevated temperatures, as well. Because some components on a substrate may not be able to withstand temperatures above a certain level, the processing time required to cure is often dictated by the component having the lowest temperature threshold. Unfortunately, lower curing temperatures typically result in significantly longer curing times.
Another disadvantage associated with conventional heating techniques is the possibility of damage resulting from the effects of thermal expansion and cooling. It is known that all materials expand or contract with a change of temperature. Thus, heating a material causes it to expand, while lowering its temperature causes it to contract. When two materials having different coefficients of thermal expansion are joined together via an adhesive resin, the two materials will expand at different rates during the application of heat to cure the resin. The two materials will also contract at different rates during cooling. The result is a build-up of stresses at the interface area between the two materials and the resin. An electronic component, which may be comprised of different materials, is especially susceptible to excessive stresses during heating and cooling, which may result in the failure of the component, or the physical separation of the component materials. Damage may be particularly acute when the materials are exposed to long heating times and elevated temperatures.
Yet another disadvantage associated with conventional heating methods is the practice of using forced air in the curing process. Because many electronic components and substrates are small in size and lightweight, moving air often times makes it difficult to maintain proper alignment during curing. Focused and defocused infrared (IR) heating techniques have been used to avoid the necessity of forced air. Unfortunately, each of these IR techniques heat both the component and the substrate on which the component is being mounted. As a result, a build-up of stresses may occur at the interface area between materials having different coefficients of thermal expansion.
The general use of microwave irradiation in combination with a curing agent is known. For example, U.S. Pat. No. 5,317,045 to Clark, Jr. et al. relates to a method of curing a polymeric material using microwave irradiation. The application of microwave irradiation decreases the time required to cure some polymers as compared with conventional heating methods. This is because the volumetric deposition of microwave irradiation is more efficient than conduction from the surface resulting from conventional heating techniques. See, for example, Polymer Curing In A Variable Frequency Microwave Oven, R. J. Lauf et al., Oak Ridge National Laboratory. See also, U.S. Pat. No. 5,296,271 to Swirbel et al., which proposes a method of curing photoreactive polymers by exposing them to microwave irradiation. Additionally, microwave processing is more economically attractive than conventional heating techniques due to the shorter processing time required to cure the resin.
The use of microwave irradiation to cure polymers, however, is not without limitations. Presently available microwave irradiation processes typically utilize a fixed frequency, such as 2.45 GHz, to cure polymers. Unfortunately, quality control and other reliability problems arise when fixed frequency microwave irradiation is used to process multiple workpieces (workpiece is generically used herein to refer to all electronic/microelectronic components and substrates subjected to microwave processing). Unless each subsequent workpiece to be irradiated is placed in substantially the same orientation and in substantially the same location within the microwave furnace as the first workpiece, the time required to cure, as well as the quality of the cure, will typically vary because of the inherent nonuniform distribution of electromagnetic energy inside microwave furnaces powered with fixed frequency microwave signals. Additionally, a particular fixed frequency may properly cure the adhesive being used to secure a component to a substrate, but the frequency may cause damage to the component being secured to the substrate, to another component on the substrate, or to another portion of the substrate as a result of localized heating or arcing.
Thus it would be desirable to provide a process for assembling electronic components which reduces the time required to cure various adhesives; allows selective curing of adhesives; allows rapid batch processing without a reduction in quality from workpiece to workpiece; reduces thermal stresses at the interface of different materials; and does not harm the electronic components or substrates.